Fix missing return value checks

Ensure that all functions have their return values checked where
appropriate. This covers all functions defined and called from within
libssl.

Reviewed-by: Richard Levitte <levitte@openssl.org>

ssl/ssl_sess.c

diff --git a/ssl/ssl_sess.c b/ssl/ssl_sess.c
index cf019c8..22d2e66 100644
--- a/ssl/ssl_sess.c
+++ b/ssl/ssl_sess.c
@@ -510,12 +510,14 @@
              */
             if (!
                 (s->session_ctx->session_cache_mode &
-                 SSL_SESS_CACHE_NO_INTERNAL_STORE))
+                 SSL_SESS_CACHE_NO_INTERNAL_STORE)) {
                 /*
                  * The following should not return 1, otherwise, things are
                  * very strange
                  */
-                SSL_CTX_add_session(s->session_ctx, ret);
+                if(SSL_CTX_add_session(s->session_ctx, ret))
+                    goto err;
+            }
         }
     }